Multiple Steam Boilers
In larger hot water systems, it is becoming a standard to do multiple boilers to better match the ever-changing loads of the building. This idea has not caught on very well for steam systems. I think it is just as important for larger steam systems to do multiple boilers to achieve maximum fuel savings. The boilers should still be sized properly according to the connected load. Measure all the radiation and duct coils and convert to square Feet of steam (EDR) and choose the boiler from that figure. All main steam piping must be insulated. You cannot half step into higher efficiency ideas. What would be the advantage of spending money for multiple boilers and have uninsulated mains making the steam prematurely convert back into condensate and must be reheated and converted back into steam. Proper sizing is still the first step in maximizing fuel savings. Multiple boilers are secondary. A point I want to make is when I say multiple boilers it means more than one boiler or in larger power burner steam boilers you can get the burners configured as low/high/low. This means the boiler will come on in a low fire, go to high and when the set point pressure is achieved it will drop back to low fire and back to high when needed. This is all automatic. Boilers that have the ability to do low/high/low will do two boilers in one, or two boilers with low/high/low is the same as four boilers when the loads are larger or many varying size zones. The idea is when the system gets hot the steam condenses in the insulated steam mains slower and the low fire tries to match the condensation rate better.
When doing multiple steam boilers, the piping is very critical. We must use a dropped header and boiler feed tanks, not condensate tanks. Each boiler must have a pump controller installed on it to start the pump and a solenoid on the return to the boiler to open and close as water is fed as the boiler requires makeup water. If two boilers are used instead of solenoid valves you may use a duplex pump. This is one tank with two pumps. One pump can be piped to each of the two boilers.
Each boiler is piped as independent boilers and the system riser from each boiler header goes above the common header and down into the common header. This will keep any condensate from one boiler when operating getting into a non-operating boiler and causing the non-operating boiler to flood.
Ignore the pipe sizes and dimensional data as this drawing is for concept only. Your pipe diameters may be different. The boilers could be controlled by different means. Two most common applications would be extra pressurtrol mounted in the piping set at different pressures or staging controls which would turn boilers on and off as needed dependent on pressure. There is one control manufacturer looks at the pipe temperature on the end of the steam main.